(a) Field of the Invention
The present invention relates to a method for establishing conditions of differential injection for manufacturing a semiconductor device.
(b) Description of the Related Art
In order to reduce a short channel effect in a deep sub-quarter micron MOSFET, extremely shallow source/drain regions having a junction depth below 50 nm is required. Low energy injection is one method of realizing such a shallow junction. However, the low injection energy reduces the amount of beam current to lower the productivity.
A differential injection method is employed as an ion injection method which has overcome the above problem. Before describing the differential injection method, an ordinary drift injection method will be described for comparison.
In a drift injecting apparatus shown in FIG. 1, an extraction electrode 10 extracts beams from an ion source 12 and injects the beams into a wafer 16 through a beam line 14. The productivity is poor when the low energy injection is employed by the drift injecting apparatus.
In an ion injection apparatus employing the differential injection method shown in FIG. 2, the extraction electrode 10 extracts beams from the ion source 12 at an energy higher than the injection energy to the wafer. The extracted beams pass through the beam line 14 and are injected to the wafer 16 immediately after the speed of the beams is reduced to that corresponding to the desired injection energy by a speed-reducing electrode 18 immediately before the injection.
A beam current having a large energy is obtained in the differential injecting apparatus to elevate the productivity because the energy supplied to the ions by the extraction electrode 10 can be made higher.
However, the differential injection is accompanied with a problem of energy contamination (refer to J. Freeman et al., IIT Conference Proceedings, 357(1992)). Since the ion beams pAss within the beam line, the ion beams interact with atoms and molecules of residual gases in the beam line. The interaction called "charge exchange" proceeds as follows.
A.sup.+ (higher speed ion)+B.sup.0 (lower speed atom).fwdarw.A.sup.0 (higher speed atom)+B.sup.+ (lower speed ion)
As a result of the interaction, a higher speed ion A.sup.+ is converted into a neutralized ion A.sup.0. The neutralized ion reaches to the wafer at an energy different from desired injection energy without being affected by an s electric field of the speed-reducing electrode. That is, it may occur that the speed of the neutralized ions is not reduced by the speed-reducing electrode and the ions are injected at the initial speed of extraction from the extraction electrode. Thus, the ions are deeply injected and the depth of the junction is made deeper.
Such neutralized ion injection is called "energy contamination" and the injected ions are called "contaminated ions". The energy contamination occurring in the differential injection causes deterioration or variations in the characteristics of the device.
The amount of the energy contamination is generally expressed as a ratio (%) between (the amount of dosage when the energy contamination exist) and (the amount of dosage when no energy contamination exist). In order to suppress the deterioration, the amount of the energy contamination should be in the range providing allowable deterioration of the device characteristics The permitted range is, for example, prescribed in the load map of SIA (Semiconductor Industry Association).
Accordingly after obtaining the permitted range of the amount of the energy contamination prescribed in the SIA load map, the device characteristics within the range can be obtained by either 1 increasing the degree of vacuum of the beam line in the ion injection apparatus or 2 reducing the distance in the beam line, to make the amount of the energy contamination at or below the permitted value.